JS: trap an output value of null
JS: trap an output value of null

An output that is null must be caught separately from an expression that
returns null.

Signed-off-by: Harry King <409587b9e6671aa0763646191d292852dc49a658@gmail.com>

jaamsim/jaamsim,jaamsim/jaamsim,jaamsim/jaamsim,jaamsim/jaamsim